Despite widespread anti-malaria interventions throughout Malawi, including country-wide distribution of insecticide treated nets (ITNs) and free treatment, the burden of malaria remains extremely high, with no measurable decrease in incidence of infection or disease over the past decade. Many factors have been hypothesized to explain such intransigence of malaria in Malawi, and in many parts of sub-Saharan Africa, such as unavailable treatment, drug resistance, improper diagnosis, silent reservoirs of infection, and failure of prevention measures. This project, which is integrated with two others in the Malawi ICEMR, addresses the epidemiology of disease risk from the perspective of human-vector interactions as influenced by ITN use and effectiveness and insecticide resistance (IR). We have designed systematic studies to understand why the burden of malaria remains high in Malawi, despite such intense efforts to reduce infection and disease. Our previous work in Malawi has shown that IR is high among major vectors such as Anopheles funestus, and that these mosquitoes are biting before and after usual sleeping time when ITNs could be useful. We hypothesize that ITN effectiveness will vary based on the intensity of transmission, IR, and human and mosquito behaviors. Hence, the goals of this proposed research are: 1) Understand how human infection and disease risk changes with ITN use and effectiveness across diverse environmental contexts and vector-human contact scenarios, 2) Determine how Plasmodium infection in vectors and humans varies with pyrethroid IR and vector behavior across three levels transmission intensity, and 3) Evaluate how human behaviors and biting behavior of Anopheles species alter the entomological inoculation rate (EIR) and human disease risk. To broach these objectives, we will undertake cohort-based, field studies of human and vector populations, measuring infection and disease incidence before, during and after the 2018 Malawi ITN mass distribution campaign. This effort allows us to evaluate the impacts of a massive experimental intervention by monitoring effectiveness of interventions and any changes in disease risk. Two cohorts will be established around a government health center in each of the three ecologically and epidemiologically different study sites. The infection cohort will allow measurement of incidence of P. falciparum infection and the disease cohort will be used to assess the incidence of malaria illness. By understanding these factors and their interactions across a range of endemic transmission settings, critical intervention failures can be identified and new strategies to prevent malaria designed. Our results should help improve intervention effectiveness in Malawi, and many other settings in sub-Saharan Africa where malaria burden remains high despite concerted efforts.